IN CORE INSTRUMENTATION FOR ONLINE NUCLEAR HEATING MEASUREMENTS OF MATERIAL TESTING REACTOR ABSTRACT

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1 IN CORE INSTRUMENTATION FOR ONLINE NUCLEAR HEATING MEASUREMENTS OF MATERIAL TESTING REACTOR C.REYNARD, J. ANDRÉ, J. BRUN, M.CARETTE, A. JANULYTE, O. MERROUN, Y ZEREGA Laboratoire Chimie Provence LCP UMR 6264 University of Provence Centre St Jérôme, Avenue Escadrille Normandie-Niemen bât Madirel, Marseille cedex 20, France A. LYOUSSI, G. BIGNAN, J-P. CHAUVIN, D. FOURMENTEL, W. GLAYSE, C. GONNIER, P. GUIMBAL, D. IRACANE, J-F VILLARD Commissariat à l Energie Atomique CEA Direction de l'energie Nucléaire Centre de Cadarache, Saint-Paul-Lez-Durance, France abdallah.lyoussi@cea.fr or christelle.carette@univ-provence.fr Keywords: Gamma heating, Calorimetry, neutron and gamma measurements, Material ageing, MTR ABSTRACT The present work focuses on nuclear heating. This work belongs to a new advanced research program called IN-CORE which means Instrumentation for Nuclear radiations and Calorimetry Online in REactor between the LCP (University of Provence-CNRS) and the CEA (French Atomic Energy Commission) - Jules Horowitz Reactor (JHR) program. This program started in September 2009 and is dedicated to the conception and the design of an innovative mobile experimental device coupling several sensors and ray detectors for on line measurements of relevant physical parameters (photonic heating, neutronic flux ) and for an accurate parametric mapping of experimental channels in the JHR Core. The work presented below is the first step of this program and concerns a brief state of the art related to measurement methods of nuclear heating phenomena in research reactor in general and MTR in particular. A special care is given to gamma heating measurements. A first part deals with numerical codes and models. The second one presents instrumentation divided into various kinds of sensor such as calorimeter measurements and gamma ionization chamber measurements. Their basic principles, characteristics such as metrological parameters, operating mode, disadvantages/advantages,... are discussed. 1. Context The growing of the energy needs due to the increasing world population associated to the fast development of large emerging countries and coupled to environmental challenge dedicated to the reduction of greenhouse gas production has contributed to the acknowledgement of the nuclear energy. In Europe this kind of energy corresponds already to an important part of the energy mix. But an improvement of the performances of nuclear power reactors is constantly required. For instance concerning the existing Generation II reactors, the question of their lifespan represents a real interest as well for safety as economical stake. Consequently, the increasing of their lifetime requires research works focusing in particular on their structure materials. The scientific aims are to better understand and to anticipate behavior of these materials under nuclear irradiation in order to determine their durability and to reach their optimization. Experimental studies on accelerated ageing of materials are carried out by means of MTR (Material Testing Reactor). The existing European MTRs, such as OSIRIS in France, BR2 in Belgium, or Halden in Norway were built principally in the sixties. Consequently they are ageing and new MTRs with higher performances are necessary. The construction of Jules Horowitz material testing Reactor (JHR) is registered in this context [1]. This MTR will succeed OSIRIS. It will be a major European research infrastructure to perform screening, qualification and safety experiments on material and fuel behaviour under irradiation for the

2 coming decades [2, 3]. It will contribute to the safety studies and to the optimization of existing or coming nuclear power reactors as well as the developments of the future ones (Generation IV reactor). The JHR core will offer specific experimental operating conditions such as high fast neutron flux to study structural material ageing and high thermal neutron flux for fuel experiments. The JHR will allow typically ~ 20 simultaneous experiences in the core and in the reflector. As of the beginning, the first experiments in the JHR will have to be performed with a high level of quality under new severe conditions. This high level will be obtained only if different criteria are respected. For instance, the first one concerns the online monitoring of the operating conditions. A real time measurement of these conditions with lowest possible uncertainty will lead to enhance accuracy and contribute to make signal interpretation more comfortable.. Then the second one deals with the ability to perform specific experimental conditions such as thermal conditions in order to avoid the influence of complex physical phenomena interferences on the experimental setup. Thus quantifying nuclear heating and especially gamma heating is of a great importance in order to design the thermo-hydraulic system, the shielding materials and structures of the reactor and of the experimental device housing the experimental load according to safety requirements and according the aim to control the temperature gradient or the stationary temperature of samples during irradiation processes. Consequently to reach this high level targeted which will guarantee the scientific quality of the conducted research programs, new experimental devices have to be designed including innovative instrumentation and associated advanced measurement methodologies[4]. According to this a new advanced research program between the LCP (University of Provence-CNRS) and the CEA (French Atomic Energy Commission) - Jules Horowitz Reactor (JHR) program deals with such scientific and technological challenge. This program called IN-CORE (Instrumentation for Nuclear radiations and Calorimetry Online in REactor) will lead to an innovative mobile device focusing to the online parametric mapping of the JHR core channels. The device will be composed by several sensors dedicated to measurements of relevant physical parameters (photonic heating, neutronic flux ). Actually in-pile instrumentation in MTRs concerns the measurements of temperature (thermocouples), dimensions (LVDT, strain gauges, hyper frequency resonant cavities), fission gas release (pressure sensor or analytical devices in laboratory), neutron flux (self powered neutron detectors, fission chambers) and gamma heating (calorimeter and gamma thermometers) [5]. In this paper we focus especially on one of relevant parameters: nuclear heating. The origin of nuclear heating will be described. Recent numerical works will be discussed. And finally, a brief state of the art related to direct and indirect measurement methods of nuclear heating phenomena in research reactor in general and MTR in particular such as calorimetry, gamma thermometer measurements, gamma ionization chamber measurements will be presented. 2. Nuclear heating and numerical models A nuclear reactor core is a place to a wide variety of nuclear reactions such as interactions of neutrons and radiations with matter. Each interaction releases an important quantity of energy that appears mainly in kinetic energy of product particles and emission of gamma rays. This energy can be absorbed locally by charged particles or transported by neutron-photons offsite of reactions and deposited in shielding material. The energy lost is transformed into thermal energy called nuclear heat. The largest part of nuclear heating of irradiated samples in a reactor core is due to gamma rays. They are created mainly by fission, capture and inelastic neutron interactions with various target nuclides. The induced gammas interact with matter via three principal modes of interaction: i) photo-electric, ii) Compton and iii) pair production effects. Thus, they communicate their energy to electrons of the medium which cede this energy in matter by ionization and excitation. This phenomenon contributes to heating which causes the temperature rises of the material exposed to radiations. This depends essentially on the materials constituting the samples and the radiation intensity to which they are exposed [6].

3 The process of calculating gamma heating is based firstly on determining the different gamma rays sources and the associated produced electrons following their interactions with matter. Secondly, one should determine the gamma flux by resolving the Boltzmann particle transport equation. The transport of electrons can be neglected if geometrical dimensions of the problem studied are larger than electron mean free path [7]. Hence, their energy is calculated at the place of their creation. Knowing that gamma heating level is proportional to gamma flux, the gamma heating rates can be obtained by multiplying gamma-ray flux by KERMA factor which represents the ratio between the energy deposited, in a given material, of incident and outgoing gamma rays. Various numerical codes or models are developed to accurately determine gamma heating as the Monte Carlo radiation transport codes: MCNPX [8] and TRIPOLI [9] and could be improved with local experimental data or the point kernel approach used in the MERCURE-5 code and the GHRRC code developed to estimate the gamma heating of small samples inside the GRR-1 research reactor core [10]. However, whatever the approach and the physical model approximation of the particle transport used, heating simulation constitutes a difficult task because various key parameters affect the computational results. Uncertainties can be induced by variance and statistics of Monte Carlo calculations but important uncertainties (about 30% (2 )) are coming from the gaps, or the gamma emission data present in the libraries of basic nuclear data [11]. Furthermore the omission of some heating mechanisms as the case of the study done by Varvayanni can affect rigorously results [10]. In other to specify and quantify these uncertainties, a set of experimental programs have been conducted for gamma heating measurements as ADAPh in the EOLE and MINERVE critical assemblies at the CEA Cadarache facility [11]. Different TLD and micro-ionization chamber for measuring the integral prompt and delayed gamma ray dose have been used in the UO2 core of MINERVE research reactor. Monte Carlo computational results obtained via TRIPOLI4-PEPIN2 are underestimated in comparison to measurements due to the lack of information concerning the data libraries used for gamma-ray emission. A Bayesian method of adjustment has been developed in order to re-estimate the principal components of the gamma heating. The studies made in this context, and detailed in the references [11, 12], seems to be promising and enhancing the possibility to reduce significantly the uncertainties on the determination of the gamma heating from 30% to 15% (2 ). Consequently, the constant improvement in modeling and simulation coupled to the requirements of new irradiation programs dedicated to fuel or material behavior characterization for various reactors (HTR [13], Gen IV systems, fusion [14]) leads to a need of a high-quality in-pile instrumentation. 3. Nuclear heating Instrumentation and measurement methods In the future Jules Horowitz Reactor in order to reduce the uncertainties in the knowledge of the experimental conditions, a selection of the most appropriate sensors for neutron and nuclear heating measurements is necessary, as well as an improvement of their performances. Nuclear heating can be measured by different methods : direct method such as calorimetry, gamma thermometer and indirect method such as gamma and neutron measurements by ionization chambers. The direct measurement of the nuclear heating in the most powerful research reactors relies on the use of calorimeters. 3.1 Calorimetry Since many years calorimetry has become a technique usually employed. On one hand for example, old scientific journals decide to change or extend their title to show their growing interest in calorimetry such as the journal created in 1969 under the name "Journal of Thermal Analysis," which in 1998 became "Journal of Thermal Analysis and Calorimetry"[15]. On the other hand various developments and research about calorimetry are performed in many different scientific fields in addition to the nuclear field such as

4 chemistry, medicine or pharmacology for the determination of thermodynamic parameters or characterization of physical phenomena [16, 17, 18]. In nuclear sciences and technologies Gunn divided radiometric calorimetry in four categories such as radionuclide calorimeters, beam calorimeters, local absorbed dose calorimeters, and in-reactor calorimeters [19, 20, 21]. The fourth category is devoted to studies of energy deposition in specific samples for engineering purposes of structural materials [22,23] or for local measurements of nuclear heating in reactor. This paper section discusses this fourth category. In nuclear reactor environment the instrumentation design is directly impacted by specific properties in addition to usual metrological characteristics such as sensitivity, low uncertainty. One of the important specific characteristic imposed by the restricted available space is the sensor compactness. In-pile calorimeters must have low dimensions (a few centimeter diameter in general) contrary to large volume calorimeters developed for radioactive element quantification such as tritium especially used in the military nuclear activities [24]. Consequently the calorimeter has also a simple robust design with few cables. In fact the calorimeter can be composed by a single cell (a core containing the material sample, a jacket and sometimes shields) [20, 22, 25] or by a twin cell (a body containing a sample cell and an empty identical cell with or without a pedestal) [21]. The sample choice leads directly to the sensor selectivity (for example graphite) because according to the core material, a specific separation of neutron- and gamma- effects (nuclear heating, gamma heating (mostly) or neutronic heating) is possible. The robustness and the reliability lead to the choice of an operating mode not requiring an auxiliary system or complex electronic supporting radiations. Various operating modes exist such as adiabatic mode, permanent mode, [21, 26] but the adiabatic one is more complicated to be implemented because it needs a pumping system to ensure a high vacuum in the inner volume to reach a good thermal isolation. Moreover with this kind of mode a lower power range can be tested by performing instantaneous measurements. Thus the heat flow operating mode based on heat transfers between the radiation absorber sample and its surroundings is preferred [22]. In fact the main calorimeters used in MTRs are pseudo-adiabatic calorimeters which have the advantage of small sizes and easy implementation (So-called Gamma-Thermometers belong to this category) and the differential calorimeters (discussed in the following paragraph). The last characteristic presented is the simplicity of the measurement method to determine the nuclear heating. On one hand, the energy deposited in the sample is determined by measuring its temperature rise with only thermocouples embedded in specific location. On second hand the calorimeter allows abstaining of the different constant s employment, hardly determinable such as thermal resistance and the nuclear heating can be determined straightforwardly thanks to an electrical calibration curve (for example a second order polynomial curve which occurs when convective and radiative heat transfers increase [22]). The heat transfer of the calorimeter under the irradiation condition is simulated with an electric heating. The relationship between the temperature difference versus the heating power due to Joule effect is used to define the nuclear heating from the temperature difference measurement during the irradiation. In the case of a differential calorimeter based on the differential measurement of the heating in a twin cell, the signal delivered can be directly proportional to the nuclear heating in the sample. Another advantage of a differential calorimeter consists on the uncertainty reduction or a sensitivity improvement. Its accuracy can be even increased by using in-situ calibration with miniaturized electric heaters. These kinds of calorimeters are largely used in MTRs for nuclear heating measurements up to 15 W/g.Another calibration method can be based on a heating curve corresponding to the irradiated step and on a cooling curve corresponding to the calorimeter removal [25

5 Figure 1: assembly of a differential calorimeter used in OSIRIS reactor for nuclear heating measurements (external diameter is about 20mm) 3.2 Ionisation Chambers Intense neutron flux can be measured online in MTRs using Self Power Neutron Detectors (SPND) or fission chambers. An SPND is made of a cylindrical emitter (for example in rhodium, cobalt, silver, vanadium, etc.) surrounded by an electric insulation (for example alumina) and a metallic body, which acts as collector. The dominant process in SPNDs is the neutron induced generation of high-energy electrons in the emitter that cross the insulator, resulting in an externally measurable current between emitter and collector. Depending on the material chosen of the emitter, a distinction between two kinds of SPNDs can be made: - prompt SPNDs, in which neutron capture gammas generate Compton and photoelectric effects, - delayed SPNDs, in which neutron capture generates short-lived isotopes emitting βrays. Figure 2: rhodium SPND for thermal neutron flux measurements (external diameter is 3mm) A fission chamber is an ionisation chamber designed for the detection of neutrons by their interaction with a solid fissile material. Most commonly, a cylindrical construction is used where the fissile material is applied as a coating on one (or both) electrodes contained in a selected filling gas. In operation, a bias voltage (in the order of hundreds of volts) is applied to the electrodes. Neutrons induce fissions in the fissile material and the high energy ionising fission products result in the generation of an electric signal between the electrodes, which is, in first approximation, linear with the neutron flux level. Different operating modes can be used for fission chambers. Among them, the Campbelling mode or Mean Square Voltage (MSV) mode allows to minimize the gamma background contribution in the signal [27]. Figure 3: sub-miniature fission chamber (external diameter is 1.5mm) Among the recent progresses in the field of neutron measurement, one can notice the development of a unique instrumentation for the online determination of the fast neutron flux, using a miniature fission chamber with 242 Pu deposit and a specific data processing [28], [29,30].

6 4. Conclusion Finally, main and central aim of IN-CORE project is a better assessment of the neutron and photon contributions in the nuclear heating in MTRs. Consequently, the instrumentation and measurement methodologies to be used in this program should allow to perform the online and simultaneous determination of nuclear heating, gamma flux, but also fast and thermal neutron flux. Different kinds of usual or innovative sensors will be selected and combined in a scanning system. This system coupled with an appropriated analytical approach of data analysis provided by direct and indirect measurements will improve the knowledge of the experimental conditions in the future Jules Horowitz Reactor and hence reducing uncertainties and enhancing accuracy of physical relevant parameters. 5. References 1. Materials subjected to fast neutron irradiation,jules Horowitz Reactor: a high performance material testing reactor, D. Iracane, P. Chaix, A. Alamo, C. R. Physique 9 (2008) Experimental material Irradiation in the Jules Horowitz Reactor, S. Carassou, G. Panichi, F. Julien, P. Yvon, M. Auclair, S. Tahtinen, P. Moilanen, S. Maire, L. Roux, TRTR-2005 / IGORR- 10 Joint meeting, September, 12-16, Neutronic study regarding transmutation fuel research at Jules Horowitz Reactor, T. Stummer, Y. Peneliau, C. Gonnier, Research Reactor Fuel Management (RRFM) Transactions Developments status of irradiation devices for the Jules Horowitz Reactor, C. Gonnier, D. Parrat, S. Gaillot, J.P. Chauvin, F. Serre,G. Laffont, A. Guigon, P. Roux, Research Reactor Fuel Management (RRFM) Transactions Innovative in-pile instrumentation developments for irradiation experiments in MTRs, J-F. Villard, IGORR Ibrahim, M.A., Heat generation and corresponding rise in temperature due to absorption of thermal neutrons in several shielding materials. Annals of Nuclear Energy, 29 : Development and validation of gamma-heating calculational methods for plutonium-burning fast reactors. Anton LÜTHI, Doctoral thesis. Ecole polytechnique federal de Lausane. 8. MCNPX, April D Monte Carlo radiation transport code. Los Alamos National Laboratory. 9. T. TRIPOLI: A general Monte Carlo code, present state and future prospects, Nimal, J.C., Vergnaud, Progress in Nuclear Energy. 24, A point kernel model for the energy deposited on samples from gamma radiation in a research reactor core, Varvayanni, M., Catsaros, N., Antonopoulos-Domis, M., Ann. Nucl. Energy 35, Qualification of a gamma-ray heating calculation scheme for the future Jules Horowitz material testing reactor (RJH), Blanchet, D., Huot, N., Sireta, P., Serviere, H., Boyard, M., Antony, M., Laval, V., Henrard, P., Ann. Nucl. Energy 35, Développement méthodologiques et qualification de schémas de calcul pour la modélisation des échauffements photoniques dans les dispositifs expérimentaux du futur réacteur d irradiation technologiques Jules Horowitz (RJH). Blanchet, D. Doctoral thesis. June Université Blaise Pascal. 13. Nuclear heating measurements for SS-316, copper, graphite, tungsten, chromium, beryllium in a copper centered assembly bombarded with 14 MeV neutrons and analysis, Y. Ikeda, F. Maekawa, M. Wada, Y. Kasugai, C. Konno, Y. Uno, A. Kumar, M.Z. Youssef b, M.A. Abdou, Fusion Engineering and Design 42 (1998) Graphite irradiation testing for HTR technology at the High Flux Reactor in Petten J.A. Vreeling, O. Wouters, J.G. Van Der Laa, Journal of Nuclear Materials 381 (2008) years anniversary, Simon, J. (2009), Journal of Thermal Analysis and Calorimetry 95(1), A survey of the year 2007 literature on applications of isothermal titration calorimetry, Bjelic, S. & Jelesarov, I. (2008), Journal of Molecular Recognition 21(5), Application des microcalorimètres aux mesures thermiques, L. Élégant, J. Rouquerol, R3010 Techniques de l Ingénieur. 18. A review of analytical applications of calorimetry, H.F. Fergusona, D.J. Fruripb, A.J. Pastorb, L.M. Peereyb, L.F. Whitingc, Thermochimica Acta 363 (2000) 'Radiometric calorimetry: A review', Gunn, S. (1964), Nuclear Instruments and Methods 29(1), 'Radiometric calorimetry: A review (1970 supplement)', Gunn, S. (1970), Nuclear Instruments and Methods 85(2),

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